When using an injection device for injecting a medical fluid such as a liquid drug into a patient's body, the injection device has to be primed, i.e., the reservoir in which the liquid drug is stored has to be de-aired. Otherwise, a risk of injecting air into the patient's blood vessels may arise which might lead to an embolism. Many injection devices therefore incorporate a means for priming the injection device. Such a priming means in general requires the injection device to be primed before a dose of the fluid to be injected can be selected. In such injection devices, a user may erroneously move the priming means back into a position which it had before priming or at the end of the priming operation which may lead to undesired accumulation of air in the reservoir.